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project-reactorflux

Java RX's Flux.merge and switchIfEmpty

I am trying to understand how JavaRx's Flux.merge and switchIfEmpty work together in regards to the below code as I am a bit confused on results I am seeing which is no doubt the result of my not fully grasping Java RX.

My question is ... If the call to wOneRepository... returns an empty list or the call to wTwoRepository... returns an empty list, will the switchIfEmpty code get executed? Or will it only get executed if both calls return an empty list?

Flux<Widget> f1 = wOneRepository.findWidgets(email).onErrorResume(error -> Flux.empty());
Flux<Widget> f2 = wTwoRepository.findWidgets(email).onErrorResume(error -> Flux.empty());
return Flux.merge(f1, f2)
                    .switchIfEmpty(Flux.error(new IllegalArgumentException("Widgets were not found")));

Thank you

Solution

  • switchIfEmpty() will only be called if the upstream Flux completes without emitting anything, and that will only happen if both f1 and f2 complete without emitting anything. So, if both findWidget calls fail, or both return empty Flux instances, or some combination of those, then switchIfEmpty will be called. If either f1 or f2 emits a Widget, then that Widget will be emitted from the merge operator, which means switchIfEmpty will not be called.